1. Field of the Invention
The present invention relates to a motorcycle, and more particularly to a motorcycle having a front wheel rotatably mounted on the front ends of upper and lower front swing arms extending horizontally and forwardly from a motorcycle body substantially parallel to each other.
2. Description of the Relevant Art
Many motorcycles have a front wheel supported by a telescopic front fork pivotally mounted on a head tube on a front, upper portion of the motorcycle frame.
Other motorcycles which do not employ the above suspension system, i.e., which have a front wheel rotatably mounted on the front ends of upper and lower front swing arms extending forwardly from a motorcycle body substantantially horizontally are disclosed in U.S. Pat. No. 4,265,329 patented May 5, 1981 and U.S. Pat. No. 4,526,249 patented Jul. 2, 1985, for example.
In the former U.S. Patent, the rear ends of the respective front swing arms are pivotally mounted for vertical swinging movement on an engine unit that doubles as a motorcycle frame. Therefore, the disclosed motorcycle has a smaller overall weight and a lower center of gravity than those of conventional motorcycles which employ vehicle frames. In the latter U.S. Patent, the rear ends of the respective front swing arms are pivotally mounted for vertical swinging movement on the downtubes of a motorcycle frame. The front swing arms have front ends pivotally mounted by respective ball joints on the upper and lower ends of a kingpin on which the front wheel is rotatably supported. The kingpin is are coupled to a front wheel steering mechanism and angularly movable about an axis interconnecting the ball joints for steering the front wheel. The axis is inclined to a vertical line, forming therewith a caster angle of the front wheel. When the motorcycle is viewed in side elevation, the front swing arms cooperate with a portion of an engine unit which joins the rear pivot points of the front swing arms and also with portions of the knuckles which join the front pivot points of the front swing arms, in forming a parallelogram. The angle at which the above axis is inclined to the vertical line remains unchanged even when the swing arms are vertically swung about the rear pivot points thereof. Therefore, the caster angle of the front wheels remain constant at all times.
In each of the conventional motorcycles disclosed in the above U.S. Patents, the engine cylinders project forwardly and upwardly from a front, upper portion of the engine body. Therefore, the rear portion of the upper swing arm is required to be arranged out of physical interference with the engine cylinders and exhaust pipes extending from front portions of the engine cylinders. As a result, the position where the rear end of the upper swing arm is pivotally coupled to the vehicle body cannot be selected with a large degree of freedom or flexibility, and the pivot point of the upper swing arm and the upper swing arm itself are complex in structure. This leads to another problem in that the pivot point of the front lower swing arm and the lower swing arm itself are also complex in structure. If a frame member were attached to a front portion of the engine unit and the rear end of the upper swing arm were pivoted to such a frame member in the motorcycle disclosed in the former U.S. Patent, then the rear pivot point of the upper swing arm could be positioned more freely. However, the upper swing arm would require a certain minimum length in order to allow the front wheel to move vertically. Therefore, if a frame member were installed on the engine unit, the wheel base would be increased. Such a problem manifests itself in the motorcycle of the latter U.S. Patent because it has steering link mechanism in the shape of a parallelogram as described above.
The present invention has been made in an effort to eliminate the aforesaid drawbacks of the motorcycles in which a front wheel is rotatably mounted on the front ends of upper and lower front swing arms extending horizontally and forwardly from a motorcycle body substantially parallel to each other.